The RibbonDialogLauncher is wired up to the RibbonCommand that you set on your group, so clicking the dialog launcher button on the group will invoke the Execute event on that group's RibbonCommand.

That being said, we will be removing RibbonDialogLauncher from the next preview and likely also from V1, so I would suggest that you use an alternative UI (for example, an additional button in the group which is labeled "More Options...").
The reason that we are removing these APIs is that we recently found out that the Dialog Launcher feature in Office is potentially infringing on a non-Microsoft patent. Office is still involved in litigation, so I'm not sure what the outcome of
the dispute will be (e.g., if Office will remove this feature from the next version and/or from the Fluent UI License), but all Microsoft Ribbon implementations are removing APIs for dialog launchers so that developers don't potentially infringe on this
patent while using our Ribbon code. At this time we do not have plans to include an alternative feature to replace the dialog launcher, so I would recommend using a different feature (such as an additional button to pop open a dialog, or a contextual
tab) in place of the dialog launcher.